Fix CSS Remarkup rule bleed

Summary: This margin rule is bleeding in some cases where we wrap phabricator-remarkup around a large amount of other content (like Dialogs). This restricts the rule to the first direct child.

Test Plan: Go to a workboard, edit a task, see proper alignment of icons.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin

Differential Revision: https://secure.phabricator.com/D14406